In the last decades magnetic resonance imaging (MRI) has proven to be a very valuable tool, inter alia in the field of medicinal diagnostics. This is especially true considering the unique features of this technique to gain a better understanding of the human body due to the possibility to obtain real-life 2- or 3-dimensional structural and functional information without the necessity to destroy or damage the tissue of interest. Inasmuch as, the technical parts has been subject to tremendous improvements, e.g. by implementation of higher magnetic field strength or more sophisticated pulse sequences, an image resolution in time and space has been achieved, which deemed impossible just a few years ago. Nevertheless, also today several sources of image artifacts are known, which might possibly limit the achievable image quality. On the one hand image artifacts may be caused by the machine setup and may include main field inhomogeneity, gradient non-linearities, timing errors and RF-interference. On the other hand several reasons for a limited image quality can be directly attributed to motions of the test subject. Here especially respiration, cardiac pulsation, blood and CSF flow, peristalsis, swallowing and voluntary motion of the subject has to be mentioned. Such test subject motions are detrimental, because typically the time scale of MRI data acquisition necessary to make an image (order of seconds) can span a temporal extent that is roughly of the order of or exceeding the time scale at which motion occurs.
One way to overcome such motion artifacts is the introduction of specific boundary conditions on the image data acquisition. The respiratory motion for instance can be countered by data acquisition during suspended breath hold. However, the clinical need for spatial resolution, spatial coverage, and temporal resolution usually puts the total acquisition time beyond the breath holding capacity of the test subjects or patients. This poses the need to reduce the resulting motion artifacts in MRI by either synchronizing the MR imaging with the physiologic signal or by acquiring MR data rapidly enough to freeze the motion.
MR imaging may be synchronized to physiologic signals either by prospective or retrospective gating. In prospective gating, the detection of a specific phase in the physiologic cycle initiates RF excitations and initiates data acquisition for a pre-defined duration. The RF excitation and data acquisition resume only after the next occurrence of the specified physiologic phase. In contrast, in retrospective gating, RF excitation and data acquisition are repeated at a fixed rate with data acquired during a user-identified physiologic phase. The recorded timing of each data acquisition in the physiologic cycle is used to compute a synthetic set of data and images are interpolated to fixed physiologic phases.
One method for accounting of the motion of a subject is disclosed in U.S. Pat. No. 5,251,629. Here a method of and an apparatus for inspecting a physical portion having physiological movement, for example, the abdominal region moving with breathing, by utilizing nuclear magnetic resonance is described. The circumstances of that area of the surface of the abdominal region which exists on a plane indicative of a slice to be imaged, is made different from the circumstances of the surface of the remaining physical portion. In this case, a person to be inspected feels a foreign matter at the abdominal region, and suppresses the movement of the abdominal region due to breathing, consciously and unconsciously, thereby moving the breast with breathing. Thus, the to-be-inspected abdominal region is kept quiet, and an accurate inspection can be made in a short period of time.
Further, US 2008/0154121 A1 describes a magnetic resonance imaging method that involves detection of a series of trigger events and acquisition of successive segments of magnetic resonance signals from respective segments of k-space. The occurrence of the next trigger event is predicted, e.g. by way of a running average, on the basis of the detected series of trigger events. Acquisition of at least one individual segment of magnetic resonance signals is triggered on the basis of the occurrence of the predicted trigger event. Triggering of the acquisition is based on the predicted trigger event, e.g. in that the instant and duration of the acquisition is adjusted on the basis of the prediction of the trigger event.
According to US 2011/0152669 A1, a magnetic resonance imaging apparatus is provided which performs myocardial perfusion imaging of an object. The apparatus comprises an imaging unit which acquires image data by imaging a heart of the object in synchronism with a biological signal from the object, and an image generating unit which generates an image concerning the heart of the object based on the image data, wherein the imaging unit applies a probe pulse for detecting body motion of the object before imaging of the heart, and applies a spatial nonselective saturation pulse before application of the probe pulse, and a local selective pulse for flipping back a flip angle of the spatial nonselective saturation pulse with regard to a region to which the probe pulse is applied.
US 2008/0309333 A1 describes a magnetic resonance system for acquiring MR data from a subject, the MR system comprising a monitoring module for monitoring a characteristic of a motion of the subject, the characteristic of the motion having a pre-determined or dynamically adjusted limit, and a pulse sequencer for applying a pulse sequence to acquire data from the subject when the characteristic of the motion is within the limit, the pulse sequence comprising at least one pulse waveform, wherein the pulse sequencer is further arranged to regulate a characteristic of the at least one pulse waveform when the characteristic of the motion surpasses the limit.
In U.S. Pat. No. 5,051,903 an apparatus for reducing image artifacts in NMR imaging is described. The apparatus matches elements of a set to values of a substantially periodic function so that the values exhibit a predetermined relationship to the elements. The matching is performed by evaluating the relative probability of the values of the substantially periodic function from the samples in the growing database of the values and assigning the values to the elements by using the evaluated relative probability, so as to maximize the probability that subsequent valves may be assigned the remaining elements according the predetermined relationship.
In “Fernandez B. et al: Adaptive trigger delay using a predictive model applied to black blood fast spin echo cardiac imaging in systole”, Proceedings of the International Society for Magnetic Resonance in Medicine, ISMRM, 17th Scientific Meeting and Exhibition, Honolulu, Hi., USA, 18-24 Apr. 2009, page 4719” it is described to acquire black blood fast spin echo in end-systolic phase by launching the double inversion recovery before the R-wave in the previous cardiac cycle.
Nevertheless, besides the above mentioned way to care about motions in MRI special requirements have to be fulfilled in specific steady state MR sequences, which are very important in the context of cine cardiac MR (CMR). One preferred sequence for cine CMR the balanced steady-state free precession (bSSFP) sequence, which is widely used for evaluating global (end-diastolic volume, end-systolic volume, and ejection fraction) and regional (wall motion, wall thickening) ventricular functions due to its superior blood-to-muscle contrast and higher intrinsic signal to noise (SNR) ratio.
The conventional bSSFP acquisition applies a pre-defined set of RF excitations during which data acquisition is disabled (dummy excitations). These dummy excitations drive the magnetization toward steady state, after which data acquisition commences. Once the steady state is reached, any interruption in the periodic application of RF pulses will drive the magnetization away from steady state and resumption of RF excitations will introduce transient signal oscillations. To avoid this, all data required for image formation is acquired immediately without any interruption by regular application of RF pulses, once the steady state is established.